A dampening volume control device is used in an offset press in order to control dampening volume. The dampening volume control device detects the dampening volume of a plate held on a plate cylinder and keeps the dampening volume in accordance with an operator's order in order to prevent a change in the quality of print caused by changing the dampening volume.
One existing dampening volume control device is shown in FIGS. 1A and 1B. The dampening volume is detected by measuring the intensity of a catoptric light reflected from the plate. Specifically, a light emitting diode (LED) radiates light towards the plate at a predetermined angle forming an irradiated area thereon. The light is reflected on the irradiated area. The catoptric light reflected from the plate is then sensed by a photo diode (PD). Generally, the intensity of the catoptric light decreases with reduction of the dampening volume as is shown in FIG. 1A, and increases with addition of the dampening volume as is shown in FIG. 1B.
The intensity of a conventional catoptric light, however, is not always directly proportional to the dampening volume. Therefore, the dampening volume can not be sensed exactly.
Japanese Patent Laid-open Publication NO. SHO 62-75305 discloses a dampening volume sensing device which helps solve this problem. The dampening volume sensing device 100 includes a first sensor 114 which is symmetrically arranged with a LED 112, a second sensor 115 which senses the unconventional catoptric light, and a third sensor 119. The dampening volume sensing device 100 senses a more accurate dampening volume than the device of FIG. 1A and 1B by adjusting the intensity of the conventional catoptric light sensed by the first sensor 114 using the intensity of unconventional catoptric light sensed by the second sensor 115.
The dampening volume sensing device of FIG. 2, however, has several disadvantages. To begin with, it requires a complex structure and configuration. With such a dampening volume sensing device, all the light reflected by the irradiated area must be sensed by the first sensor 114 and the second sensor 115. In order to do so, the third sensor 119 must be provided to set the irradiated area to an intersecting point of an axis of both the first sensor 114 and the second sensor 115. This is accomplished by adjusting the distance d between the third sensor 119 and the plate 13 until the light intensity sensed by the third sensor 119 is a maximum. Furthermore, as is shown in FIG. 3, the light intensity in a sensing point 140 differs from a light intensity in a sensing point 142 due to a grainy form.